Cut beef roast into 12 equal pieces. Place in 4-1/2 to 5-1/2-quart slow cooker. Add onions, tomatoes, potatoes, broth, garlic, thyme, salt and pepper. Cover and cook on HIGH 5 to 6 hours, or on LOW 8 to 9 hours, or until beef is fork-tender. (No stirring is necessary during cooking.)